ABSTRACT X-ray fluoroscopy provides a combination of real-time imaging, high spatial resolution, ease of use, and device compatibility that is essential for rapid diagnosis of coronary artery disease and catheter guidance during fluoroscopically-guided interventional (FGI) procedures. Although the millions of FGI procedures performed in the U.S. each year undeniably improve and save lives, these procedures entail a radiation burden on both the patient and the medical staff who perform them. Radiation risks include serious skin injury, cataracts, and cancer. Unfortunately, there is limited room for improvement in the dose efficiency of conventional x-ray systems. The 2D projection format of conventional fluoroscopy also fails to provide the 3D device and anatomic information needed for modern catheter ablation procedures. The goal of this project is to develop a novel inverse-geometry x-ray fluoroscopic system that will dramatically reduce x-ray dose to patient and staff, while simultaneously providing real-time 3D catheter guidance. The Scanning-Beam Digital X-ray (SBDX) system is a low-dose fluoroscopic/fluorographic system that performs 30 frame/sec imaging using low-scatter inverse-geometry scanning. Previous NIH-funded research enabled the construction of an advanced SBDX system capable of reducing patient entrance skin dose to 15% of a conventional dose while maintaining 100% of conventional image signal-to-noise ratio. This research also yielded two new interventional techniques that exploit the unique SBDX real-time, multiplane, tomosynthetic reconstructor: frame-by-frame 3D catheter tip tracking and calibration-free 3D vessel analysis for device sizing. This new proposal will advance SBDX into the interventional laboratory through three projects. First, a human subjects study will be performed to compare SBDX and conventional x-ray dose, image quality, and interventional device sizing accuracy. Second, a procedure-room system for 3D catheter tracking and cardiac visualization will be constructed and validated in an animal model of ablation in the left atrium. Third, the ability to perform SBDX computed tomography in the interventional room will be developed, in order to provide 3D cardiac maps for ablation procedures without the need for a separate, pre-procedure CT scan. Reducing x-ray dose in the cardiac cath lab is critical to maximizing the safety of patient and staff. Real-time three-dimensional imaging capability is needed for many modern interventional procedures. The successful conclusion of this research will be a low dose x-ray fluoroscopic system which provides clinical image quality, novel therapeutic tools, and 3D imaging capability in a single interventional laboratory.